This invention relates to a device and method for filtering certain bandwidths and reflecting the transmitted bandwidths of incident radiation, in particular solar radiation. In one embodiment shown here by way of example, the device and method filter ultraviolet sunlight and transmit and reflect all other wavelengths of sunlight and thereby provide individuals with an effective sunscreen for preventing sunburn while affording tanning.
Ultraviolet light has several beneficial effects. In addition to the cosmetic value of tanning, ultraviolet light can promote the healing of wounds and various skin diseases. Moreover, when ultraviolet light strikes the human body, it produces vitamin D, which is essential for the normal growth of bones. However, too much exposure of the body to ultraviolet light can be harmful. For example, prolonged exposure of the body to ultraviolet light can result in a reddening of the skin which is known as erythema or, more commonly, sunburn. In addition, excessive exposure of the body to ultraviolet light is a major cause of skin cancer. Moreover, excessive, long-term exposure of the body to ultraviolet light promotes severe deterioration of the epidermis. Frequently, weather-beaten, wrinkly, furrowed, splotchy, excrescence-marred faces characteristic of extreme old age develop in early middle age or sooner as a consequence of too much ultraviolet light.
Extensive studies have been made of the effects on the human skin of ultraviolet radiation in sunlight. With respect to erythema, Koller teaches that radiation between 280 and 320 nanometers produces substantially all of the erythemal (i.e., sunburn) effects and a substantial portion of the tanning effects. The maximum erythemal effect occurs at 296.7 nanometers. (L. Koller, Ultraviolet Radiation, Chap. 7, pp. 226-232, 2nd ed.) Radiation between 320 and 400 nanometers, the near ultraviolet, is pigmentogenic but not erythemogenic, and promotes tanning.
One technique for preventing harmful erythemal effects of ultraviolet light is to limit the exposure of the skin to the erythemogenic range of sunlight radiation to dosages less than those required to produce tanning. This technique, however, does not fit into the designs of a culture which enjoys outdoor activity and admires melanization of the skin through insolation.
Avoidance of exposure, however, is not necessary with the use of commercially available ultraviolet screening agents which mitigate the effects of exposure to erythemal radiation. These agents are typically in the form of a cream or lotion. When applied to the skin, these agents attenuate the dosage of erythemogenic radiation reaching the skin and thereby provide the user with some protection against the more harmful wavelengths of erythemal radiation.
The degree of protection provided by such agents is generally evaluated on the basis of exposure to summer noon-time sun for a four-hour period. To provide fast tanning with minimal protection, the agent should permit approximately 10-18 percent of the total erythemal flux of sunlight to reach the skin in the four-hour period. For regular protection and tanning, 6-12 percent of the total erythemal flux should reach the skin. For extra protection, only 1-6 percent of the total flux should reach the skin. Finally, for a total blocking effect, less than 1 percent should reach the skin.
Commercially available sunscreen agents provide these degrees of protection in various manners. For example, these agents typically comprise individual species or mixtures of chemicals such as salicylates, para-aminogenzoates, cinnamates, naphtholates, gallates, and benzophenomes in a primarily oily medium. By adjusting the concentration of the individual species or mixtures of chemicals, these agents can provide any degree of sunburn protection.
These agents, however, have several drawbacks. For example, a screening agent should demonstrate a relatively sharp absorption cutoff at 320 nanometers so that it transmits the maximum tanning radiation at wavelengths above the erythemal range. Unfortunately, the absorption spectrum of many commercially available agents extend into the tanning range and filter non-harmful tanning energies from the radiation. As a result, these agents have a very poor tanning efficiency.
Other drawbacks of these agents derive from their being in the form of lotions or creams which are applied directly to the skin. Although primarily oily, these agents are still soluble in water. Thus, perspiration of the user can dilute the agents or even completely wash them off, greatly reducing their effectiveness. In addition, the susceptibility of the chemicals in the lotion or cream to chemical and photochemical changes further decreases their ultraviolet absorbent effectiveness. Because it is difficult for the user to apply these agents to the body so that they form a continuous film of even thickness on the skin, they fail to promote uniform tanning. Moreover, the chemicals in these agents frequently exhibit unpleasant odors.
As lotions or creams, these agents are easily absorbed through the skin. As a result, they can cause pathological side effects such as irritation, allergic responses, melanosis, and trophic changes in the skin, in addition to interfering with normal growth and metabolic processes of the skin and mucous membranes and associated organs. In addition, these agents may stain or cause residual staining to skin and clothing, especially in the presence of sunlight, heat, laundry detergents, perspiration, etc.